Greenhouse gases (GHGs) generally comprise carbon dioxide that is produced primarily from combustion of fossil fuels. The accumulation of GHGs represents a significant threat to global climate stability. Atmospheric carbon dioxide has increased from about 280 ppm in the early 1800s to over 380 ppm in the early 2000s. The 2005 Intergovernmental Panel on Climate Change (IPCC) special report stated that global carbon dioxide (CO2) emissions from fossil fuel-based, large stationary sources (i.e., greater than 0.1 million tonnes of CO2 per year) totaled around 13.4 gigatonnes annually. Fossil fuels responsible for these emissions consisted of 60% coal, 11% natural gas (NG), 7% fuel oil, and a mix of others.
Although numerous strategies have been assessed for capturing and sequestering CO2 from flue gases, the most common high-volume through-put industrial systems typically route flue gases upward through vertical columns that have been provided with screens and/or packing materials over which amine-containing solvent solutions are flowed. The large surface areas of the screens and/or packings facilitate chemical reactions between the amine-containing solvents and the flue gas constituents, which usually are CO2, oxygen (O2), nitrogen (N2), nitrogen dioxide (NO2), sulfur dioxide (SO2) and fly ash of inorganic oxides. The chemical reactions result in primarily of the CO2 component of these flue gas constituents, into the solvent solutions. The solvent solutions containing absorbed CO2 are transferred to equipment that strip the absorbed gaseous flue gas constituents from the amine-containing solvents. This process recharges the amine-containing solvents which are subsequently recycled to the top of the vertical columns where they are reused.
A major problem with such systems however, is that flue gas constituents such as O2, and SO2 may participate in reactions with the amines, particularly at high amine regeneration temperatures, eventually leading to chemical degradation of the amines. Consequently, high amine costs, due to continual replacement of degraded solvents, are commonly associated with such CO2 capture and sequestration systems. Compounding these problems is the high solubility of SO2 in the solvent solutions that are commonly used in combination with amines. SO2 often remains associated with the regenerated solvent solutions when they are re-circulated back into the flue gas-routing columns wherein they subsequently induce corrosion of the packing materials and foaming of the degradation products. Attempts to ameliorate these problems include the addition of degradation inhibitors and/or scavengers, but these approaches have only achieved moderate success.
Other commercial approaches for recovering and sequestering CO2 from waste gas streams include scrubbers containing activated charcoal. Activated charcoal is particularly useful for capturing and adsorbing CO2 from waste gas streams. However, after certain periods of use, the activated charcoal becomes saturated with CO2. The saturated activated charcoal must then be replaced or alternatively, be recharged by purging with air streams containing low CO2 levels. Consequently, scrubbers comprising activated charcoal are not suitable for many industrial applications, and additionally, are burdened with high maintenance costs.
The recovered CO2 must then be compressed, transported and injected into depleted oil or gas reservoirs or into subterranean saline aquifers. Compression as high as 3500 psi is common, requiring substantial energy from the common grid or directly from the emitting facility. Combined with the energy required to regenerate chemical absorbents, this process often imposes a parasitic power loss of over twenty percent on the emitting facility, resulting in relatively poor economic justification of carbon capture and sequestration processes.
Other strategies for capturing and sequestering CO2 from atmospheric environments in order to improve air quality in buildings and/or outdoor spaces have employed biological processes. For example, plants absorb and convert copious amounts of CO2 into sugars that are used to fuel the development of plant biomass in the forms of celluloses, hemicelluloses, oligosaccharides and polysaccharides. Rapidly growing and photosynthesizing plants have been deployed for CO2 capture and sequestration in vertical plant walls installed on exteriors and interiors of buildings, and in marshland cropping systems. However, such systems are not suitable for capturing and sequestering CO2 from flue gases. Another problem with plant-based systems is the disposal of the vegetative matter at the end of the plant growth cycles.
Another biological strategy for the capture and sequestration of CO2 pertains to the use of cyanobacter microorganisms, commonly referred as photosynthetic blue-green algae. Cyanobacteria are commonly grown in pools or tanks for capture and sequestration of CO2 from atmospheres. Unfortunately, such systems are not suitable for capturing and sequestering CO2 from industrial flue gases.